Explosive lead azide process



nitc States atet 3,264,150 EXPLOSIVE LEAD AZIDE PROCESS James Paterson McNicol Leslie, Stevenston, Scotland, assignor to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain No Drawing. Filed June 24, 1963, Ser. No. 290,252 Claims priority, application Great Britain, June 28, 1962, 24,941/62 11 Claims. (Cl. 149-35) This invention relates to lead azide for use as primary initiating explosive in detonators and to processes for its production.

Lead azide for use in detonator primary initiating compositions is prepared by causing sodium azide and a water soluble lead salt to react in aqueous solution and subsequently separating the precipitate of lead azide formed. In order to modify the crystal structure of the lead azide and prevent the formation of sensitive crystals the precipitation is effected in the presence of a colloid. The colloid may be contained in the aqueous solutions of the two reactants but it is the usual practice to first prepare an aqueous solution containing 0.5-1.0 percent by weight of the colloid and to add simultaneously thereto, while the solution is agitated, strong aqueous solutions of the two reactants in approximately stoichiometric ratio. Usually the sodium azide solution is made slightly alkaline with caustic soda and contains a small amount of Rochelle salt as a crystal modifier. The lead azide containing precipitate contains 2 to 5 percent by weight of the colloid; so sufficient colloid solution must be used to provide excess colloid. In this specification the term lead azide includes lead azide treated with minor proportions of other materials to make it suitable for use as a constituent of the initiating explosive in detonators.

Dextrin is the colloid which has hitherto been preferred for the production of lead azide for commercial detonators. The use of gelatin was proposed in UK. Patent No. 142,898 but did not find commercial application because it resulted in larger crystals than were obtained by the use of dextrin in corresponding conditions and, because of the tendency of the gelatin to froth excessively, it resulted in a rather messy process.

The product precipitated from dextrin has always been found unsuitable for use as the sole constituent of primary initiating charges for detonators due to its inability to bind firm-1y under the usual loading pressures. As used in detonators it is invariably mixed with a proportion of lead trinitroresorcinate (lead styphnate), this latter ingredient being used mainly, and in the case of electric detonators solely, to impart to the mixture the necessary binding ability or cohesion. The lead trinitroresorcinate, however, is more sensitive to ignition by electrostatic discharge and has a greater tendency to adhere to the press punches than the lead azide constituent of the mixture. This latter disadvantage has been satisfactorily overcome by incorporating into the mixture a proportion of finely divided aluminium powder to act as a lubricant. One currently used mixture contains 68 parts lead azide, 29 parts lead tri-nitrorosorcinate and 3 parts aluminium, all parts being by weight. This mixture, however, is still more sensitive to ignition by electrostatic discharge than the lead azide constituent thereof.

Clearly it would be advantageous if the lead azide could be prepared in a form which was sufficiently cohesive for use as the sole ingredient of detonator primary initiating explosive, thus rendering unnecessary the inclusion in such compositions of lead trinitroresorcinate.

I have now discovered that lead azide precipitated in the presence of colloid comprising at least a proportion of gelatin treated with a suitable anti-foaming agent has the required degree of cohesion or binding ability for use as the primary initiating charge in detonators.

According to the present invention lead azide suitable for use as primary initiating explosive in detonators is prepared by effecting the formation of a lead azide precipitate in an aqueous solution containing at least 0.4 percent by weight of colloidal material of which at least a portion is gelatin treated with an anti-foaming agent, the gelatine concentration of the solution being at least 0.015 percent by weight. The lead azide precipitate may be formed by mixing together sodium azide and a water soluble lead salt for example lead nitrate. The lead azide precipitate contains 2 to 5 percent of the colloid, 1 to 3 percent lead hydroxide and incidental impurities.

In putting the process of the invention into effect we prefer to prepare an aqueous solution of the colloidal material first and to add thereto simultaneously in stoichiometric proportions strong aqueous solutions of the two reacting salts, the quantity of the total colloid used being at least equal to 5 percent of the weight of the lead azide precipitate formed. The aqueous solution of sodium azide should preferably be made slightly alkaline and contain a small proportion of Rochelle salt as ordinarily employed in lead azide precipitation.

If the colloid solution contains less than 0.4 percent of total colloid material the resulting lead azide may be excessively sensitive to ignition by electrostatic discharges or by impact or friction in presence of grit and may have poor cohesion. Solutions containing more than 1.0 percent colloid are generally rather viscous and are difficult to prepare in quantities required for commercial manufacture. In general it will be found convenient to use solutions containing about 0.75 percent of colloid. Increase in the colloid concentration beyond this value does not result in a commensurate higher pro portion of colloid being included in the lead azide composition.

Although the -total colloid may, if desired, be gelatin treated wit-h anti-foaming agent we have found that a lead azide composition having satisfactory cohesion characteristics can be prepared using colloid solutions containing only 0.015 percent gelatin the remainder of the colloid being dextrin. It is in general preferable that the gelatin should constitute not more than 60 percent by weight of the total colloid and to include at least 0.3 percent by Weight of dextrin in the colloid solution since the use of high proportions of gelatin gives larger crystals of lead azide compositions than are obtained with dextrin. The use of gelatin, however, appears to give products which are less liable to ignition by electrostatic discharge than the corresponding product obtained using dextrin so in certain cases where this hazard is especially important it may be advisable to use only gelatin or to maintain the proportion of gelatin higher than would otherwise be desirable. Where, however, the product is required to be as fine as possible the proportion of dextrin used should be high, the gelatin concentration being just slightly in excess of that required to impart the required binding ability to the product. Solutions containing 0.02-0.05 percent gelatin and 05-075 percent dextrin have been found satisfactory in these circumstances.

There is some evidence that when the colloid is constituted by a mixture of gelatin and dextrin, gelatin concentrations of about 0.2 to 0.35 percent by weight result in lead azide compositions having relatively poor cohesion characteristics and should preferably be avoided.

The gelatin used should have as high a viscosity as possible, its 12 percent aqueous jelly giving a value of at least 350 grams in the bloom gel rigidity test (British Standard 757 (1959)).

The gelatin may be treated with any of the anti-foaming agents commercially available for this purpose. Only very small quantities of anti-foaming agents are required and excess use of the agent should be avoided in case of pos- 4 i.e. the height multiplied by the weight. than 25 gram-centimeters is undesirable.

Sensitivity to electrostatic discharge (static sensitivity) .A condenser of about 330 micromicrofarad capacity A value less sible deleterious side efiects on the crystal form. We have 5 was charged to various voltaigm and discharged through found, for example, that parts per millic ll of n-o i tfi l test sariples logsely loadedbaroun ldhan electric fuselllead alcohol in the aqueous colloid solution is su cient. e crimpe into etonator tu es. e mimmum vo tage anti-foaming agent may be added to th aqueous solution for ignition in 20 tests was determined. A value in exof the colloid but it is generally more convenient to use a cess of 3000 volts is desirable. commercial gelatine preparation in which the anti-foaming 10 Crystal size.The size of the largest crystals in a test agent has been incorporated during manufacture. It is sample was determined with a microscope. Crystals also generally advantageous to include in the gelatin a larger than 200 microns are generally undesirable s1nce small proportion of a preservative material such as phenol modified charging techniques may be required to load or a phenolic derivative for exampl O-phenylphenol or its them into detonators. sodium salt. Balk density-This was determlned for a 2.2 gram Although the lead azide of the invention is suitable for iample loaded with fgentle tapping to ilqgll'dfifg tube use as primary initiating explosives by ltself, mlnor proavlng a lameter 0 one cent1meter. 1g u enslty portions of other ingredients insuflicient to destroy the is desirable. binding characteristics may if desired be included within Colloid solutions containing various amounts of dexsuch explosives. Thus for example, it may be advantrine and gelatin were used in the examples and the tageous to include some finely divided aluminum to prerespective concentrations of these ingredients are given vent particles of the explosive adhering to the press in Tables 1 and 2. punches when the charge is pressed into the detonator. The gelatin used was non-foaming gelatin (NF gela- The inclusion of aluminum also appears to improve the tin) which gave a value of 350 grams when 1ts 12 percent cohesion of the initiating explosive. aqueous jelly was tested in the bloom gel rigidity test The invention is further illustrated by the following (British Standard 757 (1959)). It contained approx examples in which all parts are by weight. mately 10 parts per milllon of n-octyl alcohol as anti- EXAMPLES 3132:1115: ssgtijrl gszlrl dlaadsinllar quantlty of sodium O-phenyl- In these examples 50 cc. of each of two solutions, one The results obtained on lead azide compositions precontaining 383 grams/liter of lead nitrate and the other r d using gelatin without dextrin in Examples 1-12, containing 150 grams sodium azide, 0.57 g a Ro ell are set forth in Table 1. For comparison the results of salt and 1.2 grams caustic soda per liter, were run simulth same t t on th mixture of 1 ad azide re ared in famously at a uniform rate i 337 f n aq the absence of a colloid are included as Example 1. It colloid solution over a period of 32 minutes. The re- 35 will be noted that the lead azide of Example 1 had good 511M112 p p Containing lead filide Was Separated cohesion but had a large crystal size, low bulk density oflawashed With Water; dried in at and and Was very sensitive to both electrostatic discharges sublectefil t0 the fOHOWIHIE tests: and to impact when mixed with grit. The progressive test gram test Charges 9 addition of gelatin to the colloid solution in Examples lead and? composltlon were Messed at 4500 1 into 40 2-12 at first reduced the cohesion until a minimum value metal detollator miles on of gram charges of was obtained at about 0.2 percent concentration but this pgntaerythgtoltetramirated (PETN) f 30 of property was restored at higher concentrations. The seni sitivity to electrostatic discharge and the grit sensitivity a on a onzcin a axls a ul'ons pd mmu also progrmsively and rapidly improved. However, al- The tlme requlred for observable defects to develop on though the crystal slze became somewhat lower it was the surface of half the charges was noted and taken as t uffic. t1 1 to b H c B tabl i mu ameasure of cohesion. A time of less than 30 minutes is no 5 Ian y OW i e genera c p m d f facture where a maximum crystal slze of 130 microns 1s consl ere unsatls actory' des' ble No of th odu ts of E ple 1 to 12 Grit sensitivity.-In this test a quantity of the lead ne 6 0 Xam s azide composition under test was mixed with about 5 would meet all reqmremnts but Pr0duc ts of Exam percent of its weight of standard sand, a 5 milligram s-am- P P 12 have exceptlonally g (1011651011 f 10W ple of the mixture was placed on a steel anvil and a steel sensltlvliy electroslatlc dlsohafge and Impact roller was placed on the sample. when contaminated with grit and would be useful where An 8.3 gram steel ball was then dropped from variou these properties are of paramount importance and a crysheights, 20 tests being made from each height and the tal size of up to 200 microns could be tolerated.

Table 1 Colloid solu- Grit Sens- Static Largest Bulk Example tiorl-NF Cohesion itivity Sensitivity Crystal Density Gelatin (per- (minutes) (gram-cm.) (Kilovolts) Size in (g./cc.)

cent) microns height observed at which half of the tests resulted in In Examples 1018 (Table 2) lead azide was prepared ignitions. The results are recorded as gram centimeters 75 using colloid solutions containing both dextrin and non- 5; foaming gelatin, the concentration of these ingredients being varied as indicated. The results obtained on testing the lead azide product are set forth in Table 2. For comparison the results obtained on lead azide pre- 6 minimum charge for initiation of PETN was about 0.06 gram.

EXAMPLE 25 pared with 0.75 percent dextrin are given under Example 5 In h Pl? a lead aZide P P f under 13 and those on initiating composition containing 3 ufacturing conditions using a colloid solution containing parts lead azide composition of Example 9, 29 parts lead Percent of gelatln Q 111 EXEIIITP16 The gelatln W trinitroresorcinate and 3 parts aluminum (ASA compothe f l as that used 111 13713111111? 24 f the lead aZlde sition) are included as Example 23. It was apparent Preclpltate P p as dtistfrlbed In t p from the results obtained that the composition of Exam- 10 The lead azide gave the following resultsple 16 prepared using only 0.02 percent of gelatin had Cohesion minutes 120 satisfactory cohesion and was markedly less sensitive Grit sensitivity, grarmcm 80 9 electrostatic discllrge than elther the ASA Sensitivity to electrostatic discharge, kilovolts 30 tion 'or the composition of Example 13 prepared With- Largest crystal Size microns 150 out gelatin. The crystal size of he pr of 15 Bulk density, gjcc, 4 ample 16 was also satisfactory and the lk denslty Was Lead azide content, percent 935 similar to that of the ASA composition.

It was again apparent from Example 19 that initial The minimum charge for PETN base charge initiation concentration of gelatin of about 0.25 percent gave a Was 0.03 gram. product which had poor cohesion so that gelatin icon- 20 What I claimisz centrations of about 0.2 to 0.35 should be avoided. 1. In the process of manufacturing lead azide for use Table 2 Colloid Solution Largest crys- Cohesion Grit Sensi- Static Sensital size in Bulk Dens- (minutes) tivity (grarntivity (Kilomicrons i y (g./cc.) NF Gelatin Dextrin cm.) volts) (percent) (percent) 0 0. 75 0. 002 0. 75 0.01 0. 75 0.02 0. 75 0.15 0. 75 0.2 0.5 0.25 0.5 0. 45 0.3 0.3 0.2 0.015 0.3 A.S.A. coniposition EXAMPLE 24 as a primary initiating explosive in a detonator by pre- In this example a lead azide was prepared under manufactuiing conditions using the colloid solution concentrations of Example 16. A commercially available nonfoaming gelatin of 350 bloom test was first prepared as a 20 per-cent gelled slab. 36 liters of a dextrin solution containing 45 grams dextrin per liter were added to 180 liters of water and the solution heated to 60 C., the solution being stirred throughout. 216 grams of the prepared 20 percent gelatin were added and dissolution occurred rapidly. While stirring was continued 32 liters of a solution containing 150 grams sodium azide, 0.57 gram Rochelle salt and 1.2 grams caustic soda per liter and 32 liters of a solution containing 383 grams/liter of lead nitrate were added simultaneously at uniform rates over a period of one hour.

The resulting precipitate of lead azide gave the following test results:

Cohesion, minutes 60 Grit sensitivity, gramacm. 40 Sensitivity to electrostatic discharge, kilovolts 5-7 Largest crystal size, microns 130 Bulk density, g./cc. 1.7 Lead azide content, percent 94.8

Detonators each containing a base charge 0.24 gram of PETN pressed at 400 p.s.i. and primary charges of 0.15 gram of the lead azide composition of this example pressed at 4000 p.s.i. were prepared and tested by firing with a commercial electric fusehead. Of 500 detonators tested all fired satisfactorily.

For satisfactory initiation of PETN detonator base charges the minimum primary charge which could be used was about 0.03 gram of the lead azide product of this example. In this respect it was markedly superior to the ASA composition of Example 23 of which the cipitating lead azide from an aqueous solution containing a water-soluble azide and a water-soluble lead salt the improvement which comprises providing in said aqueous solution at least 0.4 percent by weight of collodial material of which at least a portion is gelatin treated with an anti-foaming agent, the material being present in an amount sufiicient to provide at least 0.015 by weight percent gelatin in said solution.

2. The process of claim 1 in which the gelatin has a viscosity in its 12% aqueous jelly of at least 350 grams in the bloom gel rigidity test.

3. The process of claim 1 in which the water-soluble azide is an alkali metal azide.

4. The preparation of a lead azide of improved resistance and reduced sensitivity to electrostatic discharge by a process in accordance with claim 1 wherein at least 60 percent *by weight of the colloidal material is gelatin.

5. A process as claimed in claim 1 wherein the antifoaming agent is N-octyl alcohol.

6. A process as claimed in claim 1 wherein the gelatin contains a small proportion of a preservative material.

7. A process as in claim 1 wherein said water-soluble azide is sodium azide.

8. A process as in claim 1 wherein an aqueous solution of said colloidal material is prepared and wherein strong aqueous solutions of said water-solubl azide and said water-soluble lead salt are added simultaneously in stoichiometric proportions to said aqueous solution of colloidal material, the quantity of the total colloid being employed being at least equal to 5 percent by weight of the lead azide precipitate formed.

9. A process as in claim 8 wherein the water-soluble azide is sodium azide and wherein the strong aqueous 0 solution thereof is alkaline and contains a small proportion of Rochelle salt.

7 8 10. A process as in claim 8 wherein the colloidal me- 2,965,466 12/1960 Ball 14935 terial comprises a mixture of gelatin and dextrin, the gel&- 2,989,389 6/ 1961 Prior et al. 149-35 tin constituent being not more than 60 percent by Weight 3,095,268 6/1963 Bostrom et al 14935 X of the total colloid and the dextrin concentration in the 3,173,818 3/1965 Holloway et a1. 149-35 colloid solution being at least 0.3 percent by Weight. 5

11. A process as in claim 10 wherein the colloid solu- FOREIGN T tion contains 0.02-0.05 percent by weight of gelatin and 142,893 5/1920 Great Brl ln. 0.5-0.75 percent by weight of dextrin.

BENJAMIN R. PADGETT, Primary Exammer. References Cited by the Examiner 1O CARL D. QUARFORTH, LEON D ROSDOL,

UNITED STATES PATENTS Examiners.

2,373,800 4/1945 Acken et al. 23-101 W. T. HOUGH, A. G. BOWEN, L. A. SEBASTIAN, 2,421,778 6/1947 Fleischer et al. 14935 X Assistant Examiners. 

1. IN THE PROCESS OF MANUFACTURING LEAD AZIDE FOR USE AS A PRIMARY INITATING EXPLOSIVE IN A DETONATOR BY PRECIPITATING LEAD AZIDE FROM AN AQUEOUS SOLUTION CONTAINING A WATER-SOLUBLE AZIDE AND A WATER-SOLUBLE LEAD SALT THE IMPROVEMENT WHICH COMPRISES PROVIDING IN SAID AQUEOUS SOLUTION AT LEAST 0.4 PERCENT BY WEIGHT OF COLLODIAL MATERIAL OF WHICH AT LEAST A PORTION IS GELATIN TREATED WITH AN ANTI-FOAMING AGENT, THE MATERIAL BEING PRESENT IN AN AMOUNT SUFFICIENT TO PROVIDE AT LEAST 0.015 BY WEIGHT PERCENT GELATIN IN SAID SOLUTION. 